The present invention relates to lithium alloy batteries, and particularly to batteries having cells with electrodes formed of Li alloy/FeS or Li alloy/FeS.sub.2, having a high degree of overcharge tolerance.
Overcharge tolerance is desirable in lithium batteries, especially batteries having multiple cells, because it is usual for individual ones of a plurality of cells to have a different state of charge at a given time. Thus, when an effort is made to recharge the battery from a discharged condition, some of the cells reach their fully charged state before the other cells do. A continuation of charging beyond this point has the undesirable effect of overcharging the fully charged cells, which results in permanent damage to the battery, i.e., current collector oxidation with resulting deposition of metal particles to bridge the electrodes. Alternatively, if the charging operation is stopped when only one of the cells is fully charged, then the overall potential capacity of the battery is not realized, because all but one of the cells of the battery are only partially charged.
Attempts have been made in the past to address this problem by providing circuitry such as electronic control units for charging only the cells of the battery which have not reached their fully charged state. However, such circuitry is complex, expensive, and cumbersome. It is therefore desirable to develop an apparatus and mechanism whereby the plurality of cells of the battery may be fully charged without exposing the battery to permanent damage.
It has been proposed to develop a lithium battery with an improved overcharge capacity. For example, U.S. Pat. No. 4,324,846 suggests such a battery. The battery of U.S. Pat. No. 4,324,846 provides a negative electrode which has additional capacity at an increased lithium activity. This results in a cell having an overcharge characteristic in which the voltage level of the cell rises, during charging, steeply from about 1.5 volts to 1.8 volts. This results in a characteristic curve which is undesirable in that it does not exhibit a constant voltage output. In the arrangement described in the aforesaid patent, a tradeoff is necessary between the magnitude of the voltage discontinuity, and the amount of overcharge protection which is available.
Prior constructions of lithium alloy batteries have experienced the disadvantage of tending to form electrically conductive particles or other constituents during operation, especially during charging, through oxidation of a current collector. This adversely affects performance because the conductive constituents allow short circuit currents within the battery, which limits the available power and eliminates the capacity of that cell. It has been found that these disadvantages are caused by use of a boron nitride (an insulator) as the matrix for holding the cell's electrolyte, which can be converted to a conductive form in high lithium activity. It is therefore desirable to provide an electrolyte matrix which has less tendency to form electrically conductive constituents.
The electrolyte in previous use in lithium batteries has a relatively high melting point, which requires that the batteries be used at relatively high temperatures, over 400.degree. C. It is desirable to modify the electrolyte to allow operation at lower temperatures, as well as to increase the lithium ion content, which we have discovered will sustain a higher rate of reversible self discharge than previously appeared possible.